Download Lecture 2 : Early Cosmology

Lecture 2 :
Early Cosmology
 Getting in touch with your senses…
 Greek astronomy/cosmology
 The Renaissance (part 1)
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© Sidney Harris
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Discussion : What would an unaided
observer deduce about the Universe?
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II : GREEK COSMOLOGY
  First culture to look at world in the “modern
scientific way”
  They…
 Understood the idea of cause and effect
 Applied logic to try to understand the world
 Assumed that the Universe is fundamentally
knowable
 Sought to describe the Universe mathematically
 Understood the importance of comparing theory
with data
  BUT…
 Theoretical principles -- especially geometric
symmetry -- came first, with observations subsidiary
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The spherical Earth
 Greeks knew the Earth was a sphere!
 View of constellations changes from NS
 Observations of ships sailing over the
horizon (mast disappears last)
 Observations of the Earth’s shadow on the
Moon during lunar eclipses
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Relative size of Earth and
Moon
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Eratosthenes
  Astronomer/mathematician in
Hellenistic Egypt (c.275-195 BC)
  Calculated circumference of Earth
 Measured altitude of Sun at
two different points on the
Earth (Alexandria & Syene):
found 7° difference
 Multiplied (360°/7°)×800km to
obtain
circumference=40,000km
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Cosmology of Eudoxus and
Aristotle
  Fundamental “principles”:
 Earth is motionless
 Sun, Moon, planets and stars go around the
Earth: geocentric model
  Eudoxus (408-355 B.C.) & Aristotle (384-322 B.C.)
 Proposed that all heavenly bodies are
embedded in giant, transparent spheres that
revolve around the Earth.
 Eudoxus needed a complex set of 27
interlocking spheres to explain observed
celestial motions
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E.G., need to have 24-hr period =day and 365-day
period=year for the Sun
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http://
graduate.gradsch.uga.edu/
archive/Aristotle.html
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Dept. of Physics and
Astronomy,
Univ. of Tennesse
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Aristotle’s terrestrial physics
  Four basic elements: earth, water, air, fire
  Each element tends to move toward its “natural”
place:
  Rock (earth) in air falls, air bubble in water rises
  “Natural motions” of earthly objects are straight
lines toward center of Earth
  Bodies in motion naturally tend to come to rest on Earth
  An applied force can cause deviation from natural motion
  A body at rest on Earth will remain at rest unless a force is
applied
  Continual application of force is needed to sustain any
motion other than natural motion
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Aristotle’s celestial physics
  Heavens are governed by different laws from Earth
  Celestial bodies are composed of “ether,” a fifth
element not present on Earth
  “Natural motions” of celestial spheres are different
from terrestrial motions:
  circular, constant, and eternal
  Aristotle needed 55 spheres to explain observed motions of
Sun, Moon, planets, stars
  Space is finite, bounded by outer sphere
  But the edge is unreachable: motions become circular in
the ethereal domain
  Time is infinite
  (But why is such a perfect universe centered on such
an imperfect Earth?)
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Aristarchus of Samos (310-230 B.C.)
  Using eclipse data and geometry:
  Measured relative sizes of Earth, Moon
  Measured distance to Moon (how?)
  Attempted to measure distance to Sun
  Need to measure (using time interval ratios) the angle of
Sun when Moon is exactly at 1st or 3rd quarter
  Then use trigonometry and known Earth-Moon distance to
get Sun’s distance
  Very difficult measurement… He deduced that Sun is 20
times further from Earth than Moon… actual answer is that
Sun is 400 times further.
  First to propose a heliocentric model!
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Aristarchus Earth-Sun distance
Moon waning quarter
cos α = d/D →
D = d/cos α
d
α
Sun
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Earth
D
Moon waxing quarter
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Geocentric model
Retrograde motion
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Ptolemy’s epicycles
  Ptolemy’s original “epicycles”
  Larger circle (“deferent”) not centered
on the Earth
  Motion appeared uniform from
“equant” (offset from Earth and from
center of “deferent”)
Dept. of Physics and Astronomy Univ. of Tennesse
Needed more and more
epicycles to fit observed
angular motion
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Ptolemy and Homer…
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II : The Renaissance
 During European “dark ages,” Arab
astronomers preserved and extended
Ptolemy’s work
 Aristotelian/Ptolemaic view prevailed
in Europe, through 1400’s
 Geocentric model
 Creation at finite time in past, for
consistency with Christian theology
 Earth known to be round (Columbus
battling against flat Earthers is myth!)
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Copernicus (1473-1543)
  Nicholas Copernicus was modern founder of the
heliocentric (Sun centered) model for the solar
system
  Copernicus was born in Poland; studied in Krakow,
Bologna, Padua, Ferrara
  Rejected Ptolemy’s geocentric model because it was too
complicated
  Preferred heliocentric model with perfect circular motions
  The Copernican Principle : The Earth is not at a special
location in the Universe.
  Later, we will come across the Generalized Copernican
Principle: There is no special place in the universe, i.e.,
the universe has no center.
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Copernicus’s first work
 
Wrote and distributed to friends the Little Commentary (1514)
Axioms:
1.  There is no one center in the universe.
2.  The Earth's center is not the center of the universe.
3.  The Earth and planets revolve around the Sun.
4.  The distance from the Earth to the Sun is imperceptible compared
with the distance to the stars.
5.  The rotation of the Earth accounts for the apparent daily rotation
of the stars.
6.  The apparent annual cycle of movements of the Sun is caused by
the Earth revolving round it.
7. 
The apparent retrograde motion of the planets is caused by the
motion of the Earth from which one observes.
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Portrait from 1597
by Teothor de Bry
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Dept. of Physics and
Astronomy,
Univ. of Tennesse
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Tycho Brahe (1546-1601)
•  Flamboyant and tyranical aristocrat, but devoted
to science
•  Lived and observed on island off the coast of
Denmark
•  Last of the great “naked eye” observers
•  Made planetary observations much more
accurate than any previous… first to estimate
error bar of a measurement
•  Observed “new star” (Tycho’s supernova; 1572)
•  Demonstrated that comet was beyond Moon’s
orbit
•  From parallax observations of new star, comet:
•  knew they were not in Earth’s atmosphere
•  evidence that heavens were not immutable
•  Collected the premiere dataset on the motion
of the planets
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Next time…
 The scientific revolution:
 Kepler
 Galileo
 Newton
READING: Finish reading Chapter 2 of text
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